Epitaxy is the process of forming a single-crystalline film of material on a substrate or wafer. Generally the crystal structure or orientation of the film is the same as that of the substrate; however, the concentration and/or type of intentionally introduced impurities is usually different in the film than in the substrate. This characteristic of epitaxial films makes it possible to manufacture certain types of electronic devices, most notably integrated circuits such as Bipolar I.sup.2 L and Complementary Metal Oxide Semiconductor (CMOS) and VMOS transistor and logic circuits and discrete devices, such as Laser Diodes and High Electron Mobility Transistors.
Epitaxial films are currently commercially made by a number of techniques. The most common processes are Chemical Vapor Deposition (CVD), Physical Vapor Deposition (PVD)and Liquid Phase Epitaxy (LPE). Epitaxial growth by CVD is accomplished with reactants in the vapor phase (VPE). In CVD, gaseous molecule reactants are introduced into a reactor and undergo a chemical reaction. For example, the reactant SiH.sub.4 decomposes into a silicon atom which is adsorbed on the surface and two H.sub.2 molecules which subsequently desorb or leave the surface. The adsorbed silicon atom, although bound to the surface, can move around on the surface before becoming chemically bonded to the substrate. In epitaxial growth, the atom reaches an energetically favorable location on the surface where it can become incorporated into the crystal lattice thereby extending the substrate crystal structure into the growing Si film.
CVD is normally accomplished at reactor pressures ranging from 10.sup.-3 Torr to atmospheric pressure (760 Torr). This aspect of CVD, along with the chemical nature of the process, makes it possible to deposit uniform, conformal (i.e., good side wall and step coverage) epitaxial films on many wafers at a time.
An example of PVD is molecular beam epitaxy (MBE). In an MBE process, sources containing atoms of the material to be deposited are introduced into an ultra-high vacuum chamber at pressures typically less than 10.sup.-7 Torr. The solid sources are heated until vaporized. At these low pressures, the atoms in the vapor move with few, if any, gas phase collisions and deposit on any substrate surface they reach. When these atoms land on a hot wafer substrate surface, they behave similar to the adsorbed atoms in a CVD reactor behave and form an epitaxial film.
One advantage of physical vapor deposition (PVD) is that the ultra-high vacuum provides a clean environment for the deposition. Another advantage is that by using a mechanical shutter between the solid sources and the deposition surface very abrupt transitions can be obtained between multi-layered films of different compositions and/or doping levels. The shutter is closed until a steady-state beam of vapor is obtained from heating the solid sources. At this point the shutter is opened and deposition begins. Successive layers of films can be formed in this fashion.
The disadvantage of PVD is that conformal coverage cannot be attained due to the line-of-sight path taken by the depositing atoms, that is, shadowing effects occur. This property makes it difficult to deposit a film on more than a few wafers at a time. Furthermore, the ultra-high vacuum is expensive and difficult to maintain, thereby further reducing the efficiency and throughput of the process. With PVD, however, it is possible to deposit unique combinations of materials that cannot be deposited with CVD. PVD is thus used principally for fabricating heterojunction devices.
Conventional thermally driven CVD processes do not permit the introduction of a physical shutter system. In conventional CVD, gaseous reactants containing the elements to be deposited are injected into a chamber that contains a substrate heated to a predetermined temperature. Once the reactants are close to the substrate, they decompose, because of thermal energy, yielding the elements that form the deposit. Thermally-driven CVD is thus not capable of producing very abrupt interfaces, because of "memory" transient effects associated with the gas flow dynamics of thermally driven VPE systems.
Abrupt interfaces are desirable to be able to form layers of very thin films of material of different composition or to form narrow junctions between thin films of like composition but different degrees of doping.
Accordingly, a need exists for an epitaxial deposition process which can produce abrupt transitions between epitaxial layers of different composition or layers of like composition with different degrees of doping, but which does not require the ultra-high vacuum employed in MBE and does not incur the conformal coating difficulties associated with PVD.
The above-enumerated problems attendant to fabrication of thin films of different compositional characteristics with abrupt interfaces or thin films of like composition but different degrees of doping with abrupt junctions applies to films of elemental semiconductor material, such as Si films, or compound films, such as GaAs films, or combinations thereof.
A particular series of films, i.e., Metal Organic (MO) epitaxial films of III-IV semiconductors present an even more difficult processing problem. The wide range of compositions required, such as, AlGaAs for electrical devices and InGaAsP for optical devices, demands a crystal growth process capable of growing any combination of Al, Ga, In, As and P containing compounds. The use of these materials in optical, as well as electrical, devices requires the epilayers to have a high radiative recombination efficiency, as well as a high carrier mobility. It is also necessary that the technique provide excellent control over the thickness, uniformity, and compositions of the epilayers and over the abruptness of the interfaces between epilayers.
While LPE is capable of growing epilayers with high radiation recombination efficiency; very thin layers (less than 1000A) are difficult to produce and layer uniformity is difficult to control. Complex multiple layer heterostructures and graded composition epilayers are impractical to produce by LPE.
The epilayer composition and growth rate is controlled by the equilibrium which is established between the elemental sources and the vapor. This means that very close control of source and substrate temperatures is required for reproducible results and this is difficult to do in practice. The problems are particularly difficult with the Al source, the result being that conventional VPE is not capable of producing the important AlGaAs ternary.
Epitaxial films of Al containing compounds have been grown by MBE. Furthermore, MBE is capable of producing extremely abrupt interfaces and providing excellent control over layer thickness and uniformity. However, as previously noted, MBE suffers from several drawbacks, i.e., the necessity for ultrahigh vacuum resulting in high maintenance costs, low through-put, and conformal coating problems, making it unattractive for volume production. It is also difficult to grow phosphorous containing compounds by MBE.
Accordingly, a particular need exists for a process of producing metal organic thin film layers of III-IV compounds with abrupt change of compositional characteristics which operates at low pressures (0.1 to 5 Torr) rather than high vacuums (10.sup.-8 to 10.sup.-5 Torr) and wherein the layers have good optical and/or electrical properties.